The present invention relates to a method for improving the efficiency of a basic oxygen furnace (BOF) during steelmaking operations, and more particularly, to a method capable of effecting a reduction in the heat time of a BOF, capable of providing a reduction in the oxygen requirements and/or other raw material requirements of each heat cycle in the BOF, and/or capable of providing at least one of these advantageous reductions using materials recycled from previous BOF operations. The present invention also relates to a method for accommodating production interruptions downstream of the BOF with minimal, if any, adjustment to the rate of molten iron production at a blast furnace upstream of the BOF, by using material recycled from previous operations of the BOF to selectively increase the molten iron-to-scrap ratio demanded by the BOF.
The steelmaking industry is very competitive. Significant efforts in the steelmaking industry have been directed to increasing productivity and reducing manufacturing costs, without detrimentally affecting the quality of the steel product. Recently, consumers of steel have increased their quality requirements and have imposed stringent criteria on what they will consider to be acceptable characteristics of a steel product. The higher standards, in turn, have placed significant restrictions on productivity, and in many cases, have resulted in a need for larger quantities of raw materials. The price of the steel, however, has not kept pace with the additional requirements. Heretofore, it has been difficult to satisfy higher quality standards in a manner that does not negatively impact productivity, that does not increase operating costs (e.g., raw material costs, energy costs, labor costs, and the like), and/or that does not require capital intensive changes to the BOF or its operation.
Environmental concerns have placed additional pressure on the steelmaking industry. Recent restrictions on the handling and disposal of steelmaking byproducts (e.g., the slag and sludge produced as a result of conventional steelmaking operations) have made transportation and disposal a very expensive endeavor. This, in turn, increases the overall cost of making steel. In addition, there is slight risk of spilling whenever such materials are transported. The disposal of steelmaking byproducts also consumes landfill space, which is undesirable in view of recent efforts to reduce the rate at which landfill space is consumed.
There is consequently a need for a process of recycling the steelmaking byproducts, rather than disposing of the byproducts. Because of the other pressures faced by the steelmaking industry, there is a need for such recycling processes to have little, if any, negative impact on productivity and/or the costs associated with the steelmaking operations.
Commonly owned U.S. Pat. No. 5,785,737 discloses a method for recycling iron-bearing sludges in a steelmaking operation. Incorporated herein by reference is the subject matter disclosed in U.S. Pat. No. 5,785,737 (hereinafter the '737 patent). As disclosed in the '737 patent, wet BOF scrubber sludge (which typically has a moisture content of about 30%) can be dehydrated to produce a steelmaking revert having an improved flow rate when handled in a recycle stream. In particular, the wet sludge can be combined with hot BOF slag to provide a slag/sludge mixture. The wet sludge preferably causes the mixture to have a moisture content greater than 10% water by weight, and the hot slag, having a temperature below the molten liquid state, vaporizes the water in the mixture and reduces the moisture content to about 4% water by weight or less. The dehydrated mixture then can be recycled as a steel making revert.
Similarly, commonly owned and co-pending U.S. patent appln. Ser. No. 09/070,661, filed on Apr. 30, 1998, discloses a method for recycling iron-bearing waste material back into a BOF. Incorporated herein by reference is the subject matter disclosed in U.S. patent appln. Ser. No. 09/070,661 (hereinafter the '661 application).
Because of the foregoing problems faced by the steelmaking industry, there is a need for a method of improving the efficiency of a basic oxygen furnace (BOF) during steelmaking operations. This need extends to a method capable of effecting a reduction in the heat time of a BOF, capable of providing a reduction in the oxygen requirement and/or other raw material requirements of each heat cycle in the BOF, and/or capable of providing at least one of these reductions using materials that are recycled from previous BOF operations.
Another problem faced by the steel industry arises because of the differences between how a blast furnace operates and how a BOF operates. A blast furnace typically is most efficient when it operates continuously. A certain rate of material introduction and molten iron production is optimal. At large scale steel manufacturing facilities, it may take as much as eight hours or more for changes in blast furnace charge to have any appreciable effect on the molten iron output from the bottom of the blast furnace.
Steel production at the BOF, by contrast, is performed in batches. If there are any interruptions in the production of steel at the BOF or downstream of the BOF, a back log of molten iron from the blast furnace begins to accumulate. Environmental concerns typically make dumping of the molten iron undesirable. The molten iron, however, will begin to solidify if the interruption lasts a long time. While a reduction in the blast furnace's output seems to be a logical solution to this problem, it is seldom practical to change the blast furnace's charge material or other operating parameters to slow production and thereby compensate for the interruption. The impracticality of such an approach is evident from the delay between the adjustments and their effect on the blast furnace's output. Additional disadvantages of this approach are the inefficiencies associated with taking the blast furnace out of its optimum operating conditions.
Thus, when there is an interruption downstream of the BOF, it would be desirable to increase the BOF's demand for molten iron and decrease its reliance on scrap. This way, the back log of molten iron could be minimized, if not avoided.
There is consequently a need in the art for a method of accommodating production interruptions downstream of the BOF with minimal, if any, adjustment to the rate of molten iron production at a blast furnace upstream of the BOF. In this regard, there also is a need for a way of selectively increasing the molten-iron-to-scrap ratio demanded by the BOF. Because of the desirability of recycling steelmaking byproducts, this need extends to a method of accommodating interruptions in downstream production using material recycled from previous operations of the BOF.